1. Field of the Invention
This invention relates to a grain oriented electrical steel sheet and a production method for the same. More particularly, the invention is intended to obtain a product of a grain oriented electrical steel sheet subjected to domain refining (or subdividing) treatment and having high magnetic flux density, the product having lower iron loss than conventional values.
2. Description of the Related Art
Grain oriented electrical steel sheets are primarily employed as materials for laminated cores and coiled cores of transformers. For the purpose of reducing the power transmission and distribution cost, such a grain oriented electrical steel sheet is required to minimize energy loss caused upon power conversion, called xe2x80x9ciron loss.xe2x80x9d
Iron loss is expressed by the sum of hysteresis loss and eddy current loss. One technique for reducing hysteresis loss is to align a  less than 001 greater than  axis of an iron crystal, which is a relatively easily magnetizable axis, with the rolling direction. Thus, it is known that permeability is increased and iron loss is reduced by orienting the crystal structure of iron in a {110} less than 001 greater than  direction, called Goss orientation, at a higher concentration.
Such a crystal structure oriented in the Goss direction at a higher concentration is generally obtained by utilizing a phenomenon called secondary recrystallization. In other words, a desired structure can be produced through preferential growth of crystal grains only in the Goss direction, which is developed by utilizing abnormal grain growth with very high direction selectivity during the thermal growth process of primary recrystallized grains. On that occasion, control of two factors, i.e., direction selectivity and growth rate of abnormal grains, is important in achieving a secondary recrystallized structure oriented in the Goss direction at a higher concentration.
To that end, it is usually pursued to develop a primary recrystallized structure before secondary recrystallization in the form of a particular texture, and to form a precipitate dispersed phase, which is called an inhibitor, in a uniform and appropriate size for selectively inhibiting growth of primary recrystallized grains.
As one technique regarding an inhibitor, Japanese Examined Patent Application Publication No. 46-23820 discloses a technique for forming a composite precipitate phase of MnSe or MnS and AlN to act as a strong inhibitor. It has, however, been confirmed that even when a crystal structure oriented in the Goss direction at a higher concentration is obtained by the disclosed technique, iron loss of a product is not always reduced. The reason is that the sizes of secondary recrystallized grains are increased as the concentration in the Goss direction increases, and at the same time the so-called xcex2 angle between the grain direction [001] and the rolled surface comes closer to 0xc2x0, whereby the width of a 180xc2x0-magnetic domain is widened and eddy current loss is increased.
Various techniques have recently been proposed to reduce the magnetic domain width by an artificial method and to lower the eddy current loss. Those techniques include, for example, methods of irradiating a laser beam (Japanese Examined Patent Application Publication No. 57-2252) and a plasma flame (Japanese Unexamined Patent Application Publication No. 62-96617) in a direction substantially perpendicular to the rolling direction of a steel sheet.
With those proposed methods, the so-called xe2x80x9cstress-patternxe2x80x9d magnetic domains, which have a linear or linearly continued shape, are generated in an irradiated area by introducing thermal strains in the surface of a steel sheet. Inside such a magnetic domain, since magnetization occurs in the [100] and [010] directions, the width of a 180xc2x0-magnetic domain is reduced due to the effect of magneto-static energy caused by magnetic poles, which are generated in the boundary between the 180xc2x0-magnetic domain in the [100] direction and a stress pattern.
Also, in consideration of the fact that strain removing annealing is essential in coiled cores and the like, various methods utilizing grooves formed in a steel sheet have been developed as domain refining techniques which are endurable against the strain removing annealing. One of those techniques is represented, for example, by a method of locally forming a groove in a steel sheet after final finishing annealing so that a magnetic domain is refined due to the diamagnetic field effect produced by the formed groove. One example of methods for forming such a groove comprises the steps of locally removing an insulation coating and a primer coating by mechanical machining or irradiation of a laser beam, and then performing electrolyte etching (see Japanese Unexamined Patent Application Publication No. 63-76819).
Further, Japanese Examined Patent Application Publication No. 62-53579 discloses a method for refining a magnetic domain, wherein a steel sheet is subjected to strain removing annealing after impressing it with a gear-shaped roll, thereby achieving both groove formation and recrystallization. Additionally, Japanese Unexamined Patent Application Publication No. 59-197520 discloses a method for forming grooves in a steel sheet before finishing annealing.
It is known that, by applying those domain refining methods to a conventional grain-oriented electrical steel sheet, iron loss of a grain oriented electrical steel sheet having high magnetic flux density and having coarse crystal grains can effectively be reduced, and a value of iron loss is reduced as magnetic flux density B8 increases. However, materials exhibiting even lower iron loss are demanded under recent energy situations, whereas a drastic improvement in iron loss is difficult to achieve with conventional domain refining techniques.
On the other hand, as one of technique for improving magnetic characteristics of a grain oriented electrical steel sheet based on steel composition, it is proposed to add Cr in steel.
Aiming at reducing iron loss of a grain oriented electrical steel sheet, for example, Japanese Unexamined Patent Application Publication No. 10-259424 discloses a method of adding a predetermined amount of silicon, chromium, manganese, etc. in a hot rolled sheet to increase electrical resistivity of the hot rolled sheet to a value not lower than 45 xcexcxcexa9.cm, thereby reducing eddy current loss. Although the disclosed method proposes addition of Cr for increasing volume resistivity of a base material, it has not yet succeeded in realizing a high concentration of the grain direction and achieving a steel sheet with low iron loss, which has recently been demanded.
Also, Japanese Examined Patent Application Publication Nos. 62-54846 and 63-1371 and Japanese Unexamined Patent Application Publication Nos. 61-190017 and 2-228425 disclose techniques for adding Cr in a steel slab for the purpose of preventing deterioration of magnetic characteristics due to a change in the amount of acid-soluble Al.
Further, Japanese Unexamined Patent Application Publication Nos. 2-228425 and 5-78743, etc. disclose techniques for improving magnetic flux density with a combination of slab reheating at low temperature not higher than 1300xc2x0 C. and nitriding after decarburization annealing, wherein Cr is contained in a steel slab to widen the range of Al content in which high magnetic flux density is obtained. Japanese Unexamined Patent Application Publication No. 11-217631 also discloses a technique wherein Cr is contained in a steel slab, which is subjected to slab reheating at low temperature and nitriding. However, this Publication aims to prevent deterioration in formation of a forsterite coating.
Moreover, in the technique disclosed in Japanese Unexamined Patent Application Publication No. 10-46297, Cr is contained in a product by a similar method, i.e., a combination of slab reheating at low temperature and nitriding with NH3 gas after decarburization annealing. A primary action of added Cr is to develop a satisfactory internal coating.
Among the above-mentioned references regarding the techniques for addition of Cr, Japanese Unexamined Patent Application Publication Nos. 61-190017 makes studies as to whether the effect of reducing iron loss by domain refining is enhanced by irradiating a laser beam to a proposed grain oriented silicon steel sheet added with Cr. As a result of the studies, however, it is concluded that the effect of reducing iron loss by domain refining is hardly obtained for the steel sheet added with Cr. Any artificial domain refining techniques are not mentioned in the other references.
Meanwhile, Japanese Unexamined Patent Application Publication Nos. 9-202924, 10-130726, and 10-130727 disclose techniques for applying the domain refining treatment to a mirror-finished grain oriented electrical steel sheet. Disclosed examples include a steel sheet containing 0.12% of Cr in steel. However, those Publications neither describe the object of adding Cr, nor suggest s correlation between the addition of Cr and conditions for the domain refining treatment. Because a steel sheet is subjected to slab reheating at low temperature and nitriding, it is thought that the object of adding Cr in those Publications is the same as that in the above-mentioned techniques.
Thus, adding Cr in steel of a grain oriented electrical steel sheet is proposed as the technique for improving stability of a secondary recrystallized crystal and forming a satisfactory forsterite coating in the step of slab reheating at low temperature, or as the technique aiming at reducing iron loss of a steel sheet having low permeability with an increase in electrical resistivity. However, the concept of adding Cr with the intent to improve the effect of the domain refining treatment itself is not found in any known related art. Also, there are no references reporting any finding with respect to the relationship between conditions for the domain refining treatment and the Cr content.
Additionally, any method capable of effectively reducing iron loss of a low iron-loss steel sheet by the domain refining is not yet found, and in the present state of the art, a level of iron loss is hardly improved in comparison with a conventional one.
It would therefore be advantageous to provide a grain oriented electrical steel sheet, which has lower iron loss after domain refining than conventional values, and a production method for the same.
The inventors have studied various processes for effectively reducing iron loss after the domain refining. As a result, the inventors have discovered that the effect of the domain refining is improved beyond an expected level by adding Cr in metal part (or substrate steel or base metal; portion of a steel sheet except for a surface coating layers) of a steel sheet product and properly setting conditions for domain refining depending on the Cr content. Based on that discovery, the inventors have found a process capable of producing a steel product with lower iron loss than conventional values, and accomplished the invention.
The features of the invention are summarized below.
(I) A grain oriented electrical steel sheet with low iron loss, comprising metal part containing Si: about 2.5 to about 5.0 mass % and Cr: about 0.05 to about 1.0 mass %, and an insulation coating formed on a surface of the metal part and made up of a single layer or multiple layers, wherein, preferably, tension imparted to the metal part in the rolling direction by the insulation coating is not smaller than about 3.0 MPa, and wherein magnetic flux density (B8) satisfies formula (1), a plurality of linear strains are induced near a surface of the steel sheet and linearly extended at an angle of not larger than about 45xc2x0 (in each direction) to a direction perpendicular to a rolling direction, and an array interval D of the linear strains satisfies formula (2):
B8xe2x89xa7(2.21xe2x88x920.0604(Si)xe2x88x920.0294(Cr))xc3x970.960xe2x80x83xe2x80x83(1) 
3+5(Cr)xe2x89xa6Dxe2x89xa611+5(Cr)xe2x80x83xe2x80x83(2) 
wherein (Si) and (Cr) represent mass percentages of Si and Cr in the metal part of the grain oriented electrical steel sheet, a unit of B8 is T, and a unit of D is mm.
(II) A grain oriented electrical steel sheet with low iron loss, comprising metal part containing Si: about 2.5 to about 5.0 mass % and Cr: about 0.05 to about 1.0 mass %, and an insulation coating formed on a surface of the metal part and made up of a single layer or multiple layers, wherein, preferably, tension imparted to the metal part in the rolling direction by the insulation coating is not smaller than about 3.0 MPa, and wherein magnetic flux density (B8) satisfies formula (3), a plurality of grooves are formed in a surface of the metal part and linearly extend at an angle of not larger than about 45xc2x0 (in each direction) relative to a direction perpendicular to a rolling direction, each of the grooves having a depth (d) preferably in the range of about 1.5 to about 15% of a metal part thickness, and an array interval D of the grooves satisfies formula (4):
B8xe2x89xa7(2.21xe2x88x920.0604(Si)xe2x88x920.0294(Cr))xc3x970.960xe2x88x920.0030dxe2x80x83xe2x80x83(3) 
1+5(Cr)xe2x89xa6Dxe2x89xa68+5(Cr)xe2x80x83xe2x80x83(4) 
wherein (Si) and (Cr) represent mass percentages of Si and Cr in the metal part of the grain oriented electrical steel sheet, a unit of B8 is T, a unit of d is xcexcm, and a unit of D is mm.
(III) In the grain oriented electrical steel sheet with low iron loss of above (I) or (II), of the layers (including a single layer)making up the insulation coating, the layer in contact with the metal part is made of forsterite as a main ingredient.
(IV) In the grain oriented electrical steel sheet with low iron loss of above (I), (II) or (III), a mean length of secondary recrystallized grains in the rolling direction is not less than about 30 mm.
(V) In the grain oriented electrical steel sheet with low iron loss of any one of above (I) to (IV), the metal part further contains Bi: about 0.0005 to about 0.08 mass %.
(VI) A method of producing a grain oriented electrical steel sheet with low iron loss, comprising preparing a steel slab containing:
about 0.010 to about 0.030 mass %, in total, of one or more selected from S and Se, and one or more selected from sol. Al: about 0.015 to about 0.035 mass % and B: about 0.0010 to about 0.0150 mass %, hot rolling the steel slab to form a hot rolled sheet; obtaining a steel sheet with a final sheet thickness by, after optionally annealing a hot rolled sheet, carrying out cold rolling two or more times, including intermediate annealing one or more times, or by carrying out cold rolling once after annealing a hot rolled sheet; carrying out decarburization annealing and then final finishing annealing; applying an insulation coating agent to form an insulation coating; and carrying out flattening annealing, thereby obtaining a product, wherein the soaking temperature (T) in annealing before final cold rolling falls in a range expressed by formula (5), a plurality of linear strains are induced in a steel sheet after the flattening annealing to be linearly extended at an angle of not larger than about 45xc2x0 (in each direction) relative to a direction perpendicular to a rolling direction, and an array interval D of the linear strains satisfies a relationship of formula (2) given below:
1000xe2x88x92200(Cr)xe2x89xa6T less than 1150xe2x88x92200(Cr)xe2x80x83xe2x80x83(5) 
3+5(Cr)xe2x89xa6Dxe2x89xa611+5(Cr)xe2x80x83xe2x80x83(2) 
wherein (Si) and (Cr) represent mass percentages of Si and Cr in the metal part of the grain oriented electrical steel sheet, a unit of T is xc2x0 C., and a unit of D is mm.
(VII) A method of producing a grain oriented electrical steel sheet with low iron loss, comprising preparing a steel slab containing:
about 0.010 to about 0.030 mass %, in total, of one or more selected from S and Se, and one or more selected from sol. Al: about 0.015 to about 0.035 mass % and B: about 0.0010 to about 0.0150 mass %; hot rolling the steel slab; obtaining a steel sheet with a final sheet thickness by, after optionally annealing a hot rolled sheet, carrying out cold rolling two or more times, including intermediate annealing one or more times, or by carrying out cold rolling once after annealing a hot rolled sheet; carrying out decarburization annealing and then final finishing annealing; applying an insulation coating agent to form an insulation coating; and carrying out flattening annealing, thereby obtaining a product, wherein a soaking temperature (T) in annealing before final cold rolling falls in a range expressed by formula (5) given below, a plurality of grooves are formed in a steel sheet after the cold rolling step to linearly extend at an angle of not larger than about 45xc2x0 (in each direction) relative to a direction perpendicular to a rolling direction, and an array interval D of the grooves satisfies a relationship of formula (4) given below:
1000xe2x88x92200(Cr)xe2x89xa6Txe2x89xa61150xe2x88x92200(Cr)xe2x80x83xe2x80x83(5) 
1+5(Cr)xe2x89xa6Dxe2x89xa68+5(Cr)xe2x80x83xe2x80x83(4) 
wherein (Si) and (Cr) represent mass percentages of Si and Cr in the metal part of the grain oriented electrical steel sheet and wherein a unit of T is xc2x0 C., and a unit of D is mm.
(VIII) In the method of producing a grain oriented electrical steel sheet with low iron loss of above (VI) or (VII), the steel slab further contains Bi: about 0.001 to about 0.10 mass %.